CIENTIFIC ACHIEVEMENT
Researchers genetically engineered mammalian cells to produce their own magnetic “handles” and revealed their magnetic, physical, and chemical properties, measured in part at the Advanced Light Source (ALS).
SIGNIFICANCE AND IMPACT
The work provides a foundation for future bioengineering efforts aimed at enabling genetically controlled magnetic manipulation of molecular processes in living mammalian cells.
An internal compass for cells
Some bacteria have evolved the remarkable ability to align themselves with Earth’s magnetic field, owing to self-synthesized chains of magnetic nanocrystals that provide them with an internal compass needle. It’s thought that following magnetic field lines helps these single-celled organisms propel themselves toward optimal (for them) environments.
Multicellular organisms—including humans—can also benefit from cellular compasses, or magnetic “handles,” to maneuver cells as needed. Possible future applications include cell sorting, cell tracking (and imaging), and targeted drug delivery. However, the genetic programming that allows bacteria to natively produce magnetic organelles is lacking in mammals, and attempts to introduce magnetic agents into mammalian cells are stymied by cellular defense mechanisms.
To get around this, a large multinational research collaboration based in Germany genetically modified mammalian cells to self-produce protein nanocompartments in which iron oxides can be created and stored. The group then characterized the magnetic, physical, and chemical properties of the nanocompartment cargo and demonstrated the ability to manipulate the resulting live engineered cells using magnetic fields.
Sample synthesis and analysis
The researchers introduced into mammalian cells a set of genetic constructs (engineered DNA) for the overexpression of encapsulin, the protein building block of nanocompartment shells produced by Quasibacillus thermotolerans bacteria. Also included were constructs for a red fluorescent protein for detection purposes and a ferroxidase, an enzyme that promotes the oxidation of reactive Fe2+ to more stable Fe3+ (to facilitate iron oxide accumulation). Cellular uptake of Fe2+ was enhanced by co-expression of a protein that transports iron into cells. Finally, to provide a source of iron, the cell medium was supplemented with ferrous ammonium sulfate.
After 72 hours, the modified cells were sorted using magnetic-activated cell sorting (MACS) columns. Within the cell fraction retained in the MACS columns, the researchers discovered encapsulin shells that contained ultrafine (1–3 nm) quasicrystalline ferric oxide/hydroxide cores that exhibited ferrimagnetism and paramagnetism. However, determining the precise identity of the magnetic particles required the ability to distinguish between different species of iron oxide at the scale of individual particles.
Read more on ALS website
Image: Mammalian cells were genetically modified to synthesize protein nanocompartments in which iron oxide biomineralization takes place. The compartments, naturally produced by the bacterium Quasibacillus thermotolerans (Qt), are constructed of proteins called encapsulins. The shell size and symmetry are indicated by a triangulation number (T = 4 corresponds to a relatively large, ~43 nm shell). Co-expressed with the encapsulin was a ferroxidase (IMEF), which facilitates the accumulation of iron oxide by catalyzing oxidation of Fe2+ to Fe3+.